Using autoconf definitions instead of manually appending to CFLAGS is

smarter, and according to a contributer, it is necessary for plugin authors
using "--with-debug".

configure.in

@@ -1678,14 +1678,17 @@ if test "$with_debug" = "yes"
 then
   # Medium debug.
   AC_DEFINE([DBUG_ON], [1], [Use libdbug])
-  CFLAGS="$DEBUG_CFLAGS $DEBUG_OPTIMIZE_CC -DSAFE_MUTEX $CFLAGS"
-  CXXFLAGS="$DEBUG_CXXFLAGS $DEBUG_OPTIMIZE_CXX -DSAFE_MUTEX $CXXFLAGS"
+  AC_DEFINE([SAFE_MUTEX], [1], [Use safe mutexes])
+  CFLAGS="$DEBUG_CFLAGS $DEBUG_OPTIMIZE_CC $CFLAGS"
+  CXXFLAGS="$DEBUG_CXXFLAGS $DEBUG_OPTIMIZE_CXX $CXXFLAGS"
 elif test "$with_debug" = "full"
 then
   # Full debug. Very slow in some cases
   AC_DEFINE([DBUG_ON], [1], [Use libdbug])
-  CFLAGS="$DEBUG_CFLAGS -DSAFE_MUTEX -DSAFEMALLOC $CFLAGS"
-  CXXFLAGS="$DEBUG_CXXFLAGS -DSAFE_MUTEX -DSAFEMALLOC $CXXFLAGS"
+  AC_DEFINE([SAFE_MUTEX], [1], [Use safe mutexes])
+  AC_DEFINE([SAFEMALLOC], [1], [Use safe malloc])
+  CFLAGS="$DEBUG_CFLAGS $CFLAGS"
+  CXXFLAGS="$DEBUG_CXXFLAGS $CXXFLAGS"
 else
   # Optimized version. No debug
   AC_DEFINE([DBUG_OFF], [1], [Don't use libdbug])